Abstract:

An wafer polishing pad assembly for use in CMP includes an optical sensor
for sensing reflectivity of the wafer during polishing, and produces a
corresponding signal, and transmits the signal from the rotating pad to a
stationary portion of the assembly. The signal is transmitting off the
pad through non-contact couplings such inductive coupling or optical
couplings after being converted into signal formats enabling non-contact
transmission.

Claims:

1. A polishing pad assembly for polishing a wafer surface and collecting
and transmitting data relating to the condition of the wafer surface,
said polishing pad assembly comprising:a polishing pad;means for
directing light at the wafer surface, said means disposed within the
polishing pad;means for detecting light reflected from the wafer surface
and creating an electrical signal corresponding to the light reflected,
said means for detecting light disposed within the polishing pad;means
for processing the electrical signal corresponding to the light reflected
and producing a corresponding processed signal, said means for processing
the electrical signal disposed within the pad; anda transmitter for
transmitting the processed signal, said transmitter operably coupled to
the means for processing the electrical signal.

Description:

[0001]This application is a continuation of U.S. application Ser. No.
11/443,788 filed May 30, 2006, now U.S. Pat. No. 7,195,541, which is a
continuation of U.S. application Ser. No. 10/785,393 filed Feb. 23, 2004,
now U.S. Pat. No. 7,052,366, which is a continuation of U.S. application
Ser. No. 10/303,621 filed Nov. 25, 2002, now U.S. Pat. No. 6,695,681,
which is a continuation of U.S. application Ser. No. 09/590,470, filed
Jun. 9, 2000, now U.S. Pat. No. 6,485,354.

FIELD OF THE INVENTIONS

[0002]The inventions described below relate the field of semiconductor
wafer processing, and more specifically relates to a disposable polishing
pad for use in a chemical mechanical polishing operation performed on the
semiconductor wafers wherein the polishing pad contains an optical sensor
for monitoring the condition of the surface being polished while the
polishing operation is taking place to permit determination of the
endpoint of the process

BACKGROUND OF THE INVENTIONS

[0003]In U.S. Pat. No. 5,893,796 issued Apr. 13, 1999 and in continuation
Pat. No. 6,045,439 issued Apr. 4, 2000, Birang et al. show a number of
designs for a window installed in a polishing pad. The wafer to be
polished is on top of the polishing pad, and the polishing pad rests upon
a rigid platen so that the polishing occurs on the lower surface of the
wafer. That surface is monitored during the polishing process by an
interferometer that is located below the rigid platen. The interferometer
directs a laser beam upward, and in order for it to reach the lower
surface of the wafer, it must pass through an aperture in the platen and
then continue upward through the polishing pad. To prevent the
accumulation of slurry above the aperture in the platen, a window is
provided in the polishing pad. Regardless of how the window is formed, it
is clear that the interferometer sensor is always located below the
platen and is never located in the polishing pad.

[0004]In U.S. Pat. No. 5,949,927 issued Sep. 7, 1999 to Tang, there are
described a number of techniques for monitoring polished surfaces during
the polishing process. In one embodiment Tang refers to a fiber-optic
cable embedded in a polishing pad. This cable is merely a conductor of
light. The light source and the detector that do the sensing are located
outside of the pad. Nowhere does Tang suggest including a light source
and a detector inside the polishing pad. In some of Tang's embodiments,
fiber-optic decouplers are used to transfer the light in the optical
fibers from a rotating component to a stationary component. In other
embodiments, the optical signal is detected onboard a rotating component,
and the resulting electrical signal is transferred to a stationary
component through electrical slip rings. There is no suggestion in the
Tang patent of transmitting the electrical signal to a stationary
component by means of radio waves, acoustical waves, a modulated light
beam, or by magnetic induction.

[0005]In another optical end-point sensing system, described in U.S. Pat.
No. 5,081,796 issued Jan. 21, 1992 to Schultz there is described a method
in which, after partial polishing, the wafer is moved to a position at
which part of the wafer overhangs the edge of the platen. The wear on
this overhanging part is measured by interferometry to determine whether
the polishing process should be continued.

[0006]In conclusion, although several techniques are known in the art for
monitoring the polished surface during the polishing process, none of
these techniques is entirely satisfactory. The fiber optic bundles
described by Tang are expensive and potentially fragile; and the use of
an interferometer located below the platen, as used by Birang et al.,
requires making an aperture through the platen that supports the
polishing pad. Accordingly, the present inventor set out to devise a
monitoring system that would be economical and robust, taking advantage
of recent advances in the miniaturization of certain components.

SUMMARY

[0007]It is an objective of the present invention to provide a polishing
pad in which an optical sensor is contained, for monitoring an optical
characteristic, such as the reflectivity, of a wafer surface that is
being polished, during the polishing operation. The real-time data
derived from the optical sensor enables, among other things, the end
point of the process to be determined.

[0008]It is a further objective of the present invention to provide
apparatus for supplying electrical power to the optical sensor in the
polishing pad.

[0009]It is a further objective of the present invention to provide
apparatus for supplying electrical power for use in transmitting an
electrical signal representing the optical characteristic from the
rotating polishing pad to an adjacent non-rotating receiver.

[0010]It is a further objective of the present invention to provide a
disposable polishing pad containing an optical sensor, wherein the
polishing pad is removably connectable to a non-disposable hub that
contains power and signal processing circuitry.

[0011]In accordance with the present invention, an optical sensor that
includes a light source and a detector is disposed within a blind hole in
the polishing pad so as to face the surface that is being polished. Light
from the light source is reflected from the surface being polished and
the reflected light is detected by the detector which produces an
electrical signal related to the intensity of the light reflected back
onto the detector.

[0012]The electrical signal produced by the detector is conducted radially
inward from the location of the detector to the central aperture of the
polishing pad by a thin conductor concealed between the layers of the
polishing pad.

[0013]The disposable polishing pad is removably connected, both
mechanically and electrically, to a hub that rotates with the polishing
pad. The hub contains electronic circuitry that is concerned with
supplying power to the optical sensor and with transmitting the
electrical signal produced by the detector to non-rotating parts of the
system. Because of the expense of these electronic circuits, the hub is
not considered to be disposable. After the polishing pad has been worn
out from use, it is disposed of, along with the optical sensor and the
thin conductor.

[0014]In accordance with the present invention, electrical power for
operating the electronic circuits within the hub and for powering the
light source of the optical sensor may be provided by several techniques.
In a preferred embodiment, the secondary winding of a transformer is
included within the rotating hub and a primary winding is located on an
adjacent non-rotating part of the polishing machine. In a first
alternative embodiment, a solar cell or photovoltaic array is mounted on
the rotating hub and is illuminated by a light source mounted on a
non-rotating portion of the machine. In another alternative embodiment,
electrical power is derived from a battery located within the hub. In yet
another embodiment, electrical conductors in the rotating polishing pad
or in the rotating hub pass through the magnetic fields of permanent
magnets mounted on adjacent non-rotating portions of the polishing
machine, to constitute a magneto.

[0015]In accordance with the present invention, the electrical signal
representing an optical characteristic of the surface being polished is
transmitted from the rotating hub to an adjacent stationary portion of
the polishing machine by any of several techniques. In a preferred
embodiment, the electrical signal to be transmitted is used to frequency
modulate a light beam that is received by a detector located on adjacent
non-rotating structure. In alternative embodiments, the signal is
transmitted by a radio link or an acoustical link. In yet another
alternative embodiment, the signal may be applied to the primary winding
of a transformer on the rotating hub and received by a secondary winding
of the transformer located on an adjacent non-rotating portion of the
polishing machine. This transformer may be the same transformer that is
used for coupling electrical power into the hub, or it can be a different
transformer.

[0016]The novel features which are believed to be characteristic of the
invention, both as to organization and method of operation, together with
further objects and advantages thereof, will be better understood from
the following description considered in connection with the accompanying
drawings in which several embodiments of the invention are illustrated by
way of example. It is to be expressly understood, however, that the
drawings are for the purpose of illustration and description only and are
not intended as a definition of the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is an exploded view in perspective showing the general
arrangement of the elements of a preferred embodiment of the invention;

[0018]FIG. 2 is a front top perspective view of the optical sensor used in
a preferred embodiment of the invention;

[0019]FIG. 3 is a side elevational diagram showing an optical sensor in an
alternative embodiment of the invention;

[0020]FIG. 4 is a diagram showing a medial cross sectional view of a hub
in accordance with a preferred embodiment of the invention;

[0021]FIG. 5 is a diagram showing a medial cross sectional view of a hub
in a first alternative embodiment of the invention;

[0022]FIG. 6 is a diagram showing a medial cross sectional view of a hub
in a second alternative embodiment of the invention; and,

[0023]FIG. 7 is a diagram showing a medial cross sectional view of a hub
in a third alternative embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTIONS

[0024]The wafers with which the present invention is used are composite
structures that include strata of different materials. Typically, the
outermost stratum is polished away until its interface with an underlying
stratum has been reached. At that point it is said that the end point of
the polishing operation has been reached. The polishing pad of the
present invention is applicable to detecting transitions from an oxide
layer to a silicon layer as well as to transitions from a metal to an
oxide or other material.

[0025]Clearly, stopping a polishing machine to remove a wafer to inspect
it and then replacing the wafer into the machine and starting the machine
is a highly inefficient way of determining whether the process has been
carried far enough. Ideally, with the present invention, the polishing
process can be allowed to progress until the optical sensor of the
present invention has provided information that permits a determination
that the end point has been reached.

[0026]Although end point sensing is the main objective of the present
invention, other possibilities for using the present invention are under
consideration. These include determining how far away the end point is,
sampling various areas on a wafer, and mapping the surface of a wafer.
Although a single optical sensor is described in the following
paragraphs, it is contemplated that for some uses of the invention a
number of optical sensors may be included in a polishing pad.

[0027]The present invention involves modifying a conventional polishing
pad by embedding within it an optical sensor and other components. The
unmodified polishing pads are widely available commercially, and the
Model IC 1000 made by the Rodel Company of Newark, N.J., is a typical
unmodified pad. Pads manufactured by the Thomas West Company may also be
used. The manner in which these pads are modified in accordance with the
present invention and used will be clear from the discussion below.

[0028]In that discussion, it will be seen that the optical sensor of the
present invention senses an optical characteristic of the surface that is
being polished. Typically, the optical characteristic of the surface is
its reflectivity. However, other optical characteristics of the surface
can also be sensed, including its polarization, its absorptivity, and its
photoluminescense (if any). Techniques for sensing these various
characteristics are well known in the optical arts, and typically they
involve little more than adding a polarizer or a spectral filter to the
optical system. For this reason, in the following discussion the more
general term "optical characteristic" is used.

[0029]The words "optical" and "light" as used below include ultraviolet,
visible, and infrared types of light. The terms "radio" and "acoustic"
are used in their usual broad sense.

[0030]As shown in FIG. 1, the polishing pad 10 has a circular shape and a
central circular aperture 12. In accordance with the present invention, a
blind hole 14 is formed in the polishing pad, and the hole 14 opens
upwardly so as to face the surface that is being polished. In accordance
with the invention, an optical sensor 16 is placed in the blind hole 14
and a conductor ribbon 18, which extends from the optical sensor 16 to
the central aperture 12, is embedded within the polishing pad.

[0031]When the polishing pad is to be used, a hub 20 is inserted from
above into the central aperture 12 and secured there by screwing a base
22, which lies below the polishing pad, onto a threaded portion of the
hub 20, As best seen in FIG. 4, the polishing pad 10 is thus clamped
between portions of the hub and portions of the base. During the grinding
process, the polishing pad, the hub and the base rotate together about a
central vertical axis 24.

[0032]Also seen in FIG. 1 and FIGS. 4-7 is a non-rotating portion 26 of
the polishing machine. Preferably, it is located adjacent and above the
hub 20. Although it is not considered to be part of the present
invention, the non-rotating portion 26 is ancillary to the present
invention and its purpose will be described more fully below.

[0033]FIG. 2 is a top front perspective view showing the optical sensor
16, in a preferred embodiment, in greater detail. The optical sensor 16
includes a light source 28, a detector 30, a reflective surface 32, and
the conductor ribbon 18. The conductor ribbon 18 includes a number of
generally parallel conductors laminated together for the purpose of
supplying electrical power to the light source 28 and for conducting the
electrical output signal of the detector 30 to the central aperture 12.
Preferably, the light source 28 and the detector 30 are a matched pair.
In general, the light source 28 may be a light emitting diode and the
detector 30 is a photodiode. The central axis of the bundle of light
emitted by the light source 28 is directed horizontally initially, but
upon reaching the reflective surface 32 the light is redirected upward so
as to strike and reflect from the surface that is being polished. The
reflected light also is redirected by the reflective surface 32 so that
the reflected light falls on the detector 30, which produces an
electrical signal in relation to the intensity of the light falling on
it. The arrangement shown in FIG. 2 was chosen to conserve the height of
the sensor.

[0034]As smaller light sources and detectors become available, it may be
possible to dispense with the reflective surface 32 and instead to use
the arrangement shown in side view in FIG. 3.

[0035]The optical components and the end of the conductor ribbon 18 are
encapsulated in the form of a thin disk 34 that is sized to fit snugly
within the blind hole 14 of FIG. 1. In the arrangements of FIGS. 2 and 3,
it is understood that baffles may be used to reduce the amount of stray
light reaching the detector.

[0036]Included within the conductor ribbon 18 are at least three
conductors: a power conductor 36, a signal conductor 38, and one or more
return or ground conductors, not shown.

[0037]As best seen in FIG. 4, the power conductor 36 terminates adjacent
the central aperture 12 of the polishing pad 10 at a power plug 40, and
the signal conductor 38 likewise terminates at a signal plug 42. When the
hub 20 is inserted into the central aperture 12, the power plug 40 makes
electrical contact with the power jack 44, and the signal plug 42 makes
electrical contact with the signal jack 46. An O-ring seal 48 prevents
the liquids used in the polishing process from reaching the plugs and
jacks. Ajar lid type of seal 50 is provided in the base 22 to further
insure that the electronic circuits within the hub remain uncontaminated.

[0038]An electrical signal produced by the detector 30 and related to the
optical characteristic is carried by the conductor 52 from the signal
jack 46 to a signal processing circuit 54, that produces in response to
the electrical signal a processed signal on the conductor 56 representing
the optical characteristic. The processed signal on the conductor 56 is
then applied to a transmitter 58.

[0039]In the embodiment shown in FIG. 4, the transmitter 58 applies a
time-varying electrical current to the primary winding 60 of a
transformer that produces a varying magnetic field 62 representative of
the processed signal. The magnetic field 62 extends upward through the
top of the hub 20 and is intercepted by a secondary winding 64 of the
transformer which is located on an adjacent non-rotating portion 26 of
the polishing machine, or on some other non-rotating object. The varying
magnetic field 62 induces a current in the secondary winding 64 that is
applied to a receiver 66 that produces on the terminal 68 a signal
representative of the optical characteristic. This signal is then
available for use by external circuitry for such purposes as monitoring
the progress of the polishing operation and/or determining whether the
end point of the polishing process has been reached.

[0040]A similar inductive technique may be used to transfer electrical
power from the adjacent non-rotating portion 26 of the polishing machine
to the rotating hub 20. A prime power source 70 on the non-rotating
portion 26 applies an electrical current to the primary winding 72 of a
transformer that produces a magnetic field 74 that extends downward
through the top of the hub 20 and is intercepted by a secondary winding
76 in which the varying magnetic field induces an electrical current that
is applied to a power receiver circuitry 78. The power receiver 78
applies electrical power on the conductor 80 to the power jack 44, from
which it is conducted through the power plug 40 and the power conductor
36 to the light source 28. The power receiver 78 also supplies electrical
power to the signal processing circuit 54 through the conductor 82, and
to the transmitter 58 through the conductor 84. At present, the magnetic
induction technique is the best mode and preferred embodiment for
transferring power into the rotating hub 20. In one embodiment the
winding 60 is the same winding 76, and the winding 64 is the same winding
72. The superimposed power and signal components are at different
frequency ranges in this embodiment and are separated by filtering.

[0041]FIGS. 5-7 show alternative embodiments in which other techniques are
used to transfer signals from the rotating hub 20 to a non-rotating
portion 26 of the polishing machine, and to transfer electrical power
from the non-rotating portion 26 into the rotating hub 20.

[0042]In the embodiment shown in FIG. 5, the transmitter 58 further
includes a modulator 86 that applies to a light emitting diode or laser
diode 88 a frequency modulated current representative of the processed
signal that represents the optical characteristic. The light-emitting
diode 88 emits light waves 90 that are focused by a lens 92 onto a
photodiode detector 94. The detector 94 converts the light waves into an
electrical signal that is demodulated in the receiver 96 to produce on
the terminal 68 an electrical signal representative of the optical
characteristic. At present, this is the best mode and preferred technique
for transferring the electrical signal from the rotating hub 20 to the
non-rotating portion 26 of the polishing machine.

[0043]Also, in the embodiment of FIG. 5, the prime source of electrical
power is a battery 98 that supplies power to a power distribution circuit
100 that, in turn, distributes electrical power to the power jack 44, to
the signal processing circuit 54, and to the transmitter circuit 58.

[0044]In the embodiment of FIG. 6, the transmitter 58 is a radio
transmitter having an antenna 102 that transmits radio waves 104 through
the top of the hub 20. The radio waves 104 are intercepted by the antenna
106 and demodulated by the receiver 103 to produce an electrical signal
on the terminal 68 that is representative of the optical characteristic.

[0045]Also in the embodiment of FIG. 6, electrical power is generated by a
magneto consisting of a permanent magnet 110 located in the non-rotating
portion 26 and an inductor 112 in which the magnetic field of the
permanent magnet 110 induces a current as the inductor 112 rotates past
the permanent magnet 110. The induced current is rectified and filtered
by the power circuit 114 and then distributed by a power distribution
circuit 116.

[0046]In the embodiment of FIG. 7, the transmitter 58 further includes a
power amplifier 118 that drives a loudspeaker 120 that produces sound
waves 122. The sound waves 122 are picked up by a microphone 124 located
in the non-rotating portion 26 of the polishing machine. The microphone
124 produces an electrical signal that is applied to the receiver 126
which, in turn, produces an electrical signal on the terminal 68 that is
representative of the optical characteristic.

[0047]Also in the embodiment of FIG. 7 electrical power is generated in
the rotating hub 20 by a solar cell or solar panel 128 in response to
light applied to the solar panel 128 by a light source 132 located in the
non-rotating portion 26. The electrical output of the solar panel 128 is
converted to an appropriate voltage by the converter 134, if necessary,
and applied to the power distribution circuit 116.

[0048]Thus, there has been described a polishing pad, for use in a
chemical mechanical polishing operation, containing an optical sensor for
monitoring the condition of the surface that is being polished, during
the polishing operation. The polishing pad, including the optical system,
is disposable, and is used with a non-disposable hub that contains
circuitry for receiving the signal produced by the optical sensor, for
processing the signal and for transmitting the signal to a non-rotating
station. The hub also contains circuitry for supplying power to the
optical sensor as well as to the other electronic circuits located in the
hub. In the several embodiments described above, it is seen that the
signal may be transmitted from the rotating hub to the non-rotating
station by radio waves, sound waves, light waves, or by magnetic
induction. Also, in the various embodiments, power may be supplied by
including a battery in the hub or by coupling electrical power into the
hub through a solar panel activated by externally applied light or by a
magneto in which a stationary permanent magnet induces a current in an
inductor that is mounted on the rotating hub.

[0049]The foregoing detailed description is illustrative of several
embodiments of the invention, and it is to be understood that additional
embodiments thereof will be obvious to those skilled in the art. The
embodiments described herein together with those additional embodiments
are considered to be within the scope of the invention.